1. Field of the Disclosure
This disclosure generally relates to an image system and, more particularly, to a pixel circuit with low power consumption, an image system including the same and an operating method thereof.
2. Description of the Related Art
Referring to FIG. 1, it is a schematic diagram of a conventional bipolar junction transistor (BJT) pixel circuit 100. The BJT pixel circuit 100 includes a photodiode 11, a BJT transistor 13, a pixel capacitor Cp, a power source VDD, a plurality of switching elements SWres, SWshut, SW0, SW0d and two source Followers SF and SFd.
FIG. 2 is an operational timing diagram of an image sensor including the BJT pixel 100 of FIG. 1, and herein the sensor array having three pixel circuits P1, P2 and P3 is taken as an example for illustration purpose.
The BJT pixel circuit 100 may operate in two phases, including a reset phase and a shutter phase, by controlling the switching elements, wherein in the reset phase the switching elements SWres are conducted, while in the shutter phase the switching element SWshut is conducted. The reset phase is used to release charges left in the photodiode 11 and to charge the pixel capacitor Cp to an initial voltage Vint. The shutter phase is to allow the BJT pixel circuit 100 to discharge the initial voltage Vint and the discharging rate is a function of light intensity. FIG. 1 shows a state of the shutter phase which operates corresponding to a system light source 20. The system light source 20 is activated corresponding to a first frame A and deactivated corresponding to a second frame B (referring to FIG. 2), wherein the second frame B is sometimes referred to an ambient image frame.
In the first frame A, a period between the system light source 20 being activated (e.g. time t1) and the switching element SWshut being conducted (e.g. time t2) is referred to a pre-flash time, which is arranged to allow the BJT transistor 13 to achieve a stable output. After the switching element SWshut is conducted, charges held in the pixel capacitor Cp discharges via the BJT transistor 13 in an exposure time (e.g. time t2 to t3) to lower the initial voltage Vint, wherein a decrement of the initial voltage Vint corresponds to the light intensity received by the photodiode 11.
However, in the pre-flash time, the system light source 20 is turned on previously and thus the total power consumption of the system is increased.